Wall Street expects Apple's Mac unit shipments to drop 4% year-over-year this quarter to 2.2 million units, according to Piper Jaffray's Gene Munster. That is an unbelievable drop in a year. Last year, Apple's March quarter Mac shipments grew 51% year-over-year.

The quarter is only halfway over, but early reports don't look good. Research firm NPD Group, which measures U.S. retail sales, says Apple's Mac unit sales dropped 6% year-over-year in January. Its Mac revenues dropped faster: 11%, according to NPD.

What happened? A combination of several things: A huge drop in the PC market led by the U.S. financial crash; a stale desktop Mac lineup; and a perking up in one area of the PC market where Apple doesn't participate -- so called "netbooks," or cheap, low-end, ultra-portable laptops.

What about that cratering PC market? We think it's unlikely Apple will lower prices across the board. That's not Apple's style -- we assume that even now, they would sacrifice unit sales for margins.

But that -- combined with a slowing iPod business, too -- means slower growth for Apple. Wall Street only expects Apple to grow revenues 6% year-over-year this quarter, a massive drop from a year ago, when revenues grew 43% year-over-year.

Which at least partially explains why Apple is still trading at a lower multiple (17 times its last 12 months' earnings) than some of its big tech peers, such as Google (26x) or Amazon (42x).